The present invention relates to analytical furnaces and particularly to an improved electrode structure of a resistance furnace and the combination of the electrode structure with a resistance crucible.
Analytical furnaces heat specimens, such as chips and pin samples, and the like, typically ranging in mass from about 1 mg to about 1 gram in a resistive crucible, typically made of graphite. Resistance furnaces employ such graphite crucibles which are clamped between two electrodes to pass an electrical current through the crucible, heating specimens to temperatures of 2500° C.-3000° C. or higher. The gaseous byproducts of fusing the specimen are then swept by an inert gas, such as helium, through the furnace system to an analyzer for the subsequent analysis of the specimen gases of interest using suitable detectors. Such an analyzer is represented by Model ONH836, which is commercially available from Leco Corporation of St. Joseph, Mich.
The heating of a specimen in existing graphite crucibles requires a significant amount of electrical power, in the neighborhood of 7500 watts. This, in turn, requires the electrodes coupled to the graphite crucible to be water cooled. Even with water cooling and the use of tungsten/copper alloy tipped electrodes, the electrodes tend to wear. The use of replaceable tips, such as disclosed in U.S. Pat. No. 4,419,754, although protecting the body of the electrode, still require frequent replacement since, even though water cooled, the electrode tips can be subjected to temperatures as high as 1050° C. during use. Also, existing crucible geometries, such as represented by U.S. Pat. Nos. 3,636,229, 3,899,627, and 4,328,386, when heated to temperatures approaching or exceeding 3000° C., tend to have hot spots instead of uniformly heated floors. Also, their side walls allow a specimen, which is heated to a bubbling state, to overflow the crucible and, in some cases, contaminate the upper electrode.
There exists a need, therefore, to improve the performance of existing resistance furnaces, including reducing power consumption, improving electrode lifetime, and providing a crucible which, when used with the electrodes, fuses a variety of specimen shapes efficiently without contaminating the upper electrode.